We propose using complementary in vivo and in vitro models to explore the mechanisms underlying high altitude (HA)-induced compensatory lung growth. Normal postnatal alveolar growth is driven by mechanical lung strain imposed by the growing rib cage and involves activation of epidermal growth factor (EGF) axis. There is evidence that hypoxia enhances alveolar growth independent of mechanical lung strain, but may retard rib cage growth and prolong the duration of postnatal somatic maturation. Based on this idea, we will define the effect of chronic HA exposure on normal lung-rib cage interaction during maturation, and examine the regulatory role of the EGF axis in HA-induced lung growth; these issues have never been examined. Hypotheses are: 1) Immature animals raised at HA show an initial alveolar growth spurt that, by reducing alveolar septal stress from the outward recoil of the rib cage, attenuates normal developmental signals. Subsequently, net rate of lung growth declines to that governed by growth rate of the rib cage but at a higher lung volume. 2) Duration of lung growth is prolonged at HA, due to delayed skeletal epiphyseal union and/or continued alveolar growth after epiphyseal union. 3) Inhibition of rib cage growth rate at increasing altitudes sets the upper limit of lung dimensions achievable in a manner dependent on the animal's age, the altitude and the duration of exposure. 4) Activation of EGF axis by hypoxia is an important pathway mediating HA-induced alveolar growth. Weanling guinea pigs will be raised at 3,l00 m, 3,800 m, 4,500 m or at low altitude (LA, 160 m) for up to 7 mo.; epiphyseal union and maximum lung size are normally reached by 5 mo. at LA. Separate adult animals will be exposed to HA to determine the maturity-dependence of response. Resting tidal volume and ventilation will be measured regularly. Terminally, pressure-volume curves of lung and thorax are measured followed by lung sampling and detailed morphometric analysis of acinar structures, including estimates of lung diffusing capacity, membrane diffusing capacity and pulmonary capillary blood volume. Bony epiphyses and rib dimensions are examined. Alveolar tissues are probed for expressions of EGF, its receptor (EGF-R) and EGF-R activation. Specific effects of hypoxia on the EGF axis and epithelial cell growth and maturation will be isolated in vitro using human fetal lung explant and type II cell cultures in the presence or absence of EGF analogs or specific EGF-R inhibitors. Cell growth is assessed from DNA and protein synthesis, cell number, volume and surface area; maturation is assessed from major surfactant protein content and volume of lamellar bodies.